Biaxially oriented polyester films have been used in various applications because of their excellent thermal properties, dimensional stability, mechanical properties, and ease of control of surface morphology and, in particular, are well known to be useful as a substrate, for example, for a magnetic recording medium that is highly strengthened using stretching techniques. In recent years, in magnetic recording media such as a magnetic tape, higher-density recording for weight saving of materials, miniaturization, and increase in capacity has been demanded. For the higher-density recording, it is effective to use a shorter recording wavelength and a smaller recording track. However, a smaller recording track has a problem in that deviation of the recording track is likely to occur because of heat during tape running and deformation due to temperature/humidity changes during tape storage. Accordingly, there has been an increased demand for improvement of properties such as dimensional stability in the transverse direction in the usage environment and storage environment of a tape.
From this standpoint, aromatic polyamides with high rigidity which are superior to biaxially oriented polyester films in strength and dimensional stability have been sometimes used for a substrate. However, aromatic polyamides are expensive and costly, and are not practical as a substrate in a versatile recording medium.
There have been disclosed techniques for reducing a coefficient of hygroscopic expansion using a polymer alloy or copolymerization to improve dimensional stability in the transverse direction of a polyester film (JP 2010-37448 A, JP 2010-31116 A and JP 2009-221277 A).
However, the techniques disclosed in JP '448, JP '116 and JP '277 have problems in that slittability deteriorates; breakage is likely to occur during film formation; and the like.
It could therefore be helpful to provide an excellent biaxially oriented polyester film, particularly, a biaxially oriented polyester film that undergoes little dimensional change due to environmental changes when used as a magnetic recording medium and has excellent storage stability and good slittability, film forming ability, and processing suitability.\